How much does a house extension cost? We get asked this question a lot, so, we thought it worthy of its own post!
It’s not an easy question to answer and obviously depends on a number of factors (etc, etc…), but that’s not the answer you’re looking for is it? You’re really looking for a very rough figure, a general guideline, a starting point. OK, I’ll try my best.
Let’s start with a basic single storey extension
say 4m x 5m
You may have heard that the rule of thumb is £1,000-£1,200 per m2. However, this figure is now somewhat out of date. From experience, £1,500 per m2 is a now a more realistic estimate for a basic / mid-range house extension in 2019. In London and some parts of the South East, these figures could be between £1,800-2,200 per m2. See our separate post on house extension costs in London and the South East here.
So, for your 4 x 5m extension, the build cost would be roughly £30,000.
- Add on 10-15% for professional fees (architect, planning application, building regs, structural engineer) (£4,500 at 15%)
Single storey house extension cost = £40,500
How about a double storey extension, same footprint?
As a rule of thumb you can add 50% extra to the build cost of a single storey extension.
So, £30,000 +50% = £45,000 for the basic build cost
- Add on 10-15% for professional fees (architect, planning application, building regs, structural engineer) (£6,750 at 15%)
Double storey house extension cost = £60,750
The above figures would apply to a typical, uncomplicated extension in most parts of the UK. However, you should consider that these figures would creep up if any of the following apply:
Does it include a kitchen or bathroom?
- Add say, £5,000 for a bathroom. Costs may be more or (if you are very thrifty) slightly less depending on your sanitary ware and finishes.
Add £10,000 for a kitchen (again, completely depends on your specification, £10,000 should get you a lower-mid range kitchen with appliances)
Finishes and fittings
Above costs assume modest finishes, fittings, electrics and lighting. Think simple painted walls, carpet floors or possibly engineered oak.
- If you’re wanting higher-end finishes e.g. tile finishes, fitted joinery etc the costs will increase.
Glazing will increase the cost also. Sliding or folding doors are a popular addition to extensions at the moment. Expect £5,000+ for around a 2.5m width opening.
Are there any complex site constraints?
Your builder will need to include additional costs in their quotation if any of the following apply:
- Your soil type and ground conditions may present more costly construction methods / techniques.
Limited access to the site. Will it be easy for the builder to bring in tools and machinery easily?
Existing structure. Are you removing any structural walls? If so there will likely be additional costs for steelwork.
Are you moving any pipework, drainage, gas meters?
- As already mentioned above, in the South East you can expect to pay more than the rest of the UK, and notably more in London.
Materials and construction techniques
- Consider the type/ quality of construction materials involved and the type of construction.
Managing or reducing your project costs
1. It pays to get professional advice from a good architect. They can help you realise the main ambitions for your extension, whilst helping control your costs. They will work with you to make sure you get the most out of your budget to recommend where your budget is best spent, and where costs would be unnecessary or could be minimised.
2. Make sure you get 3-5 quotes from recommended builders (your architect can help compile a tender list for you) to get a firm idea of the costs before starting work.
Your architect can also help you with your building contract, to prevent any nasty surprises down the line. They can also inspect the builder’s work to ensure it’s being carried out in accordance with the agreed design and specifications.
Finding the right architect
Choosing the right architect for your home project is a critical first step, but can also be a bit of a minefield. This is exactly why I set up this website in the first place!
As well as being founder of designforme.com, I am also a qualified architect. I found that many homeowners search online to find well established or high-end practices (it makes sense – these companies have bigger marketing budgets with stylish websites). However, such practices are often too busy, too expensive and, in fact, often less experienced in dealing with small residential projects on ‘normal’ budgets.
So I wanted to create this platform for those design professionals I know who are suitable, experienced and eager to take on domestic projects like yours. They are often small, young practices or freelancers. The problem was that, before Design for Me, they were very difficult to find! All you need to do is tell us a bit about your project…
Emily Design for Me